The principle of an atomic or molecular beam resonator consists of detecting a resonance in a hyperfine energy state of the atom or molecule in order to obtain a standard frequency. For this purpose the particles are emitted in vapour form from a source and then concentrated into a narrow beam by a collimator. The beam of particles passes through a first energy state selecting magnet, commonly referred to as the A magnet, which selects the particles having a certain first energy state. These particles are directed to a microwave interaction module in which they are subjected firstly to a constant magnetic field, commonly referred to as the C field, and secondly to an interrogation magnetic field. When the frequency of this interrogation magnetic field is equal to a resonance frequency of the particles, the latter undergo a transition from the first energy state to a second certain energy state. The beam of particles emerging from the microwave interaction module then passes through a second state selector, commonly referred to as the B magnet, which selects the particles having the second energy state to direct them to a detector. The number of particles detected is used for producing a command signal of the frequency of an oscillator which emits, to the microwave interaction module, an electrical signal generating the interrogation magnetic field. In this way an oscillator is obtained the frequency of which is controlled by the resonance frequency of the particles.